I'm having the same issue, all automation jobs run, send emails etc, just the back up will not process nor can I do a manual download of it. this is the error:
Cron Job: ERROR : Unable to unlock tables: HY000 2014 Cannot execute queries while other unbuffered queries are active. Consider using PDOStatement::fetchAll(). Alternatively, if your code is only ever going to run against mysql, you may enable query buffering by setting the PDO::MYSQL_ATTR_USE_BUFFERED_QUERY attribute.
Nothing in the setup has changed other than the system went on pHp7. All was working perfect until the php upgrade.
Any suggestions/ideas?
Bill